Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150586

150586 - The Detailed Synthesis Process Determines Thermomechanical Behaviors of a Liquid Crystal Elastomer 

Liquid crystal elastomers (LCEs) are rubbery polymer networks that are responsive to external stimuli including heat and light. They present a nematic phase with directionally ordered liquid crystal molecules (mesogens) at room temperature and an isotropic phase with no ordering at high temperature, enabling large thermo-induced deformation. During their deformation in the nematic phase, mesogens can re-orient towards the stretching direction, leading to large stretch with minimum stress, a phenomenon called “soft elasticity” [1]. These unique features of LCEs make them promising candidates as soft robotic actuators [2] and energy absorbing [3] materials in emerging applications. However, despite the rapid advances in the experiment and modeling of LCEs, a critical question has remained unanswered: how does the detailed synthesis process of an LCE determine its behavior under a prescribed mechanical load and temperature? This study combines material synthesis, characterization, and theoretical modeling to investigate the effect of synthesis process on thermomechanical behaviors of various types of LCEs. Experimentally, we modify the well-established two-stage polymerization and synthesize monodomain and polydomain LCEs under isotropic- or nematic-genesis conditions. We characterize their spontaneous deformation and uniaxial stress-stretch behaviors at different temperatures. We identify two types of distinct behaviors in nematic-genesis LCEs and relate them to behaviors of isotropic-genesis polydomain samples. Inspired by the experiment, we develop a constitutive model [4, 5] consisting of two polymer networks and their network non-ideality. The theoretical model successfully captures thermomechanical behaviors of various LCEs at different temperatures. Finally, we discuss limitations of the current model in capturing the mechanical hysteresis and nematic order parameter of LCEs.

 

1.           Bai, R. and K. Bhattacharya, Photomechanical coupling in photoactive nematic elastomers. Journal of the Mechanics and Physics of Solids, 2020. 144.

2.           Li, Y., et al., Three-dimensional thermochromic liquid crystal elastomer structures with reversible shape-morphing and color-changing capabilities for soft robotics. Soft Matter, 2022. 18(36): p. 6857-6867.

3.           Azoug, A., et al., Viscoelasticity of the polydomain-monodomain transition in main-chain liquid crystal elastomers. Polymer, 2016. 98: p. 165-171.

4.           Wei, Z. and R. Bai, Temperature-modulated photomechanical actuation of photoactive liquid crystal elastomers. Extreme Mechanics Letters, 2022. 51: p. 101614.

5.           Wei, Z., P. Wang, and R. Bai, Thermomechanical Coupling in Polydomain Liquid Crystal Elastomers. Journal of Applied Mechanics, 2024. 91(2).

 

Presenting Author: Zhengxuan Wei Northeastern University

Presenting Author Biography: A fourth year PhD Candidate in Mechanical Engineering at Northeastern University. Studying the mechanics of soft materials such as liquid crystal elastomers, hydrogels and ionogels.

Authors:

Zhengxuan Wei Northeastern University
Umme Hani Bootwala Northeastern University
Ruobing Bai Northeastern University